Methods and apparatus for recycling tail gas in syngas fermentation to ethanol

ABSTRACT

The invention present provides a method (and suitable apparatus) to convert biomass to ethanol, comprising gasifying the biomass to produce raw syngas; feeding the raw syngas to an acid-gas removal unit to remove at least some CO 2  and produce a conditioned syngas stream; feeding the conditioned syngas stream to a fermentor to biologically convert the syngas to ethanol; capturing a tail gas from an exit of the fermentor, wherein the tail gas comprises at least CO 2  and unconverted CO or H 2 ; and recycling a first portion of the tail gas to the fermentor and/or a second portion of the tail gas to the acid-gas removal unit. This invention allows for increased syngas conversion to ethanol, improved process efficiency, and better overall biorefinery economics for conversion of biomass to ethanol.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/751,981 filed Jan. 24, 2020, which is a continuation of U.S. patentapplication Ser. No. 15/876,198 filed Jan. 21, 2018, which claims thebenefit of U.S. Provisional Patent Application 62/518,295 filed Jun. 12,2017, the entirety of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the field of processes,process configurations, and apparatus for the conversion of synthesisgas to products, such as ethanol.

BACKGROUND OF THE INVENTION

Synthesis gas (hereinafter referred to as syngas) is a mixture ofhydrogen (H₂) and carbon monoxide (CO). Syngas can be produced, inprinciple, from virtually any material containing carbon. Carbonaceousmaterials commonly include fossil resources such as natural gas,petroleum, coal, and lignite; and renewable resources such aslignocellulosic biomass and various carbon-rich waste materials. It ispreferable to utilize a renewable resource to produce syngas because ofthe rising economic, environmental, and social costs associated withfossil resources.

Syngas is a platform intermediate in the chemical and biorefiningindustries and has a vast number of uses. Syngas can be converted intoalkanes, olefins, oxygenates, and alcohols. These chemicals can beblended into, or used directly as, diesel fuel, gasoline, and otherliquid fuels. Syngas can be converted to liquid fuels, for example, bymethanol synthesis, mixed-alcohol synthesis, Fischer-Tropsch chemistry,and syngas fermentation to ethanol. Syngas can also be directlycombusted to produce heat and power.

It is known that certain microorganisms can ferment combinations ofcarbon monoxide, hydrogen, and carbon dioxide to ethanol according tothe following overall reactions:

6 CO+3 H₂O→C₂H₅OH+4 CO₂

6 H₂+2 CO₂→C₂H₅OH+3 H₂O

Fermentation according to these reactions often employs anaerobicconditions. Depending on the organism and reaction conditions (e.g.,pH), various other products can be produced, such as acetic acid,butyric acid, or butanol. Some strains of anaerobic microorganisms arereported to convert syngas to ethanol, n-butanol, or other chemicalswith high selectivity.

Syngas fermentation to products such as ethanol and acetic acid canachieve fairly high selectivity, but due to mass-transfer limitationsand low activities per unit volume of reactors, the reactors tend to bevery large. Syngas conversion in well-mixed reactors is generallylimited.

Additionally, production of ethanol from syngas will result in theco-formation of CO₂. This CO₂ is present in the tail gas of thefermentor, i.e. a vapor stream deriving from the fermentor. The tail gasgenerally contains any unconverted syngas, the CO₂ produced in thefermentation, and the inerts (including CO₂) initially contained in thesyngas feed to the fermentor. The tail gas is commonly burned to recoverthe energy in the unconverted syngas as well as the energy in any othercombustible components contained in the conditioned syngas stream, suchas methane.

The unconverted syngas cannot simply be recycled to extinction. Theinerts and the CO₂ produced in the fermentor must be removed from theoverall process. Removal of CO₂ from the tail gas in a separate unitdownstream of the fermentor is relatively expensive. Also, separation ofthe unconverted syngas from the inert gases and other species (such asmethane) is not desirable.

In view of these problems associated with syngas fermentation, what isneeded is an improved process configuration that more efficientlyutilized syngas components to produce liquid products of interest, suchas ethanol. Preferably, any such improvements do not cause significantincreases in overall plant capital costs.

SUMMARY OF THE INVENTION

In some variations, the invention provides a method of converting acarbonaceous feedstock to a syngas-fermentation product, the methodcomprising:

(a) introducing a carbonaceous feedstock and an oxidant to a gasifier,under suitable gasification conditions to produce a raw syngas streamcomprising CO, H₂, and CO₂;

(b) optionally feeding at least a portion of the raw syngas stream to asyngas-cleanup unit, to produce an intermediate syngas stream;

(c) feeding at least a portion of the raw syngas stream and/or at leasta portion of the intermediate syngas stream, if present, to an acid-gasremoval unit, to remove at least some of the CO₂ and produce aconditioned syngas stream;

(d) feeding at least a portion of the conditioned syngas stream to afermentor, under suitable fermentation conditions and in the presence ofsuitable microorganisms and nutrients to biologically convert one ormore of CO, H₂, or CO₂ to a syngas-fermentation product;

(e) capturing a tail gas from an exit of the fermentor, wherein the tailgas comprises at least CO₂ and unconverted CO or H₂;

(f) recycling a first amount of the tail gas to the fermentor in anamount described by R₁, the volumetric ratio of the first amount to thetail gas, wherein R₁ is selected from 0 to 1; and

(g) recycling a second amount of the tail gas to the acid-gas removalunit in an amount described by R₂, the volumetric ratio of the secondamount to the tail gas, wherein R₂ is selected from 0 to 1,

wherein R₁+R₂ is greater than 0; and

wherein R₁+R₂ is not greater than 1.

The carbonaceous feedstock may include, or consist essentially of,biomass. The oxidant may include one or more of air, oxygen, and steam.The gasifier is a fluidized-bed gasifier, in some embodiments.

In some embodiments, the method includes feeding at least a portion ofthe raw syngas stream to a syngas-cleanup unit, to produce anintermediate syngas stream. The acid-gas removal unit may be configuredto additionally remove at least some H₂S, if present.

In some embodiments, the first amount of the tail gas is compressedbefore being recycled to the fermentor. In these or other embodiments,the second amount of the tail gas is compressed before being recycled tothe acid-gas removal unit. Optionally, the first amount and the secondamount of the tail gas are separately compressed before being recycledto the fermentor and the acid-gas removal unit, respectively.

In some embodiments, R₁ is selected from 0 to about 0.5, or from 0 toabout 0.2. In some embodiments, R₂ is selected from 0 to about 0.8, orabout 0.2 to about 0.5. In some embodiments, the sum R₁+R₂ is selectedfrom about 0.001 to about 0.8, such as from about 0.25 to about 0.5.

The method may include a tail gas recycle control strategy to respond toone or more upstream parameters selected from the group consisting offeedstock type, oxidant profile, syngas-generation design orperformance, syngas-cleanup design or performance, and acid-gas removaldesign or performance.

The method may include a tail gas recycle control strategy to respond toone or more fermentor parameters selected from the group consisting oftemperature, pressure, residence time, pH, redox potential, nutrientconcentration, cell viability, and cell vitality. Some embodimentsfurther include recycling cells from the fermentor back to the gasifier.

The method may include a tail gas recycle control strategy to respond toone or more fermentor variables selected from the group consisting of COconversion, H₂ conversion, CO₂ conversion, ethanol selectivity, ethanolproductivity, ethanol titer, and acetic acid selectivity.

In certain embodiments, the method includes a tail gas recycle controlstrategy to control the CO₂ content in the feed to the fermentor. Forexample, the CO₂ content in the feed to the fermentor may be controlledto a level selected from about 5 vol % to about 50 vol %, such as about10-40 vol % or about 20-30 vol % CO₂, by adjusting R₁ and/or R₂.

In certain embodiments, the method includes a tail gas recycle controlstrategy to control the acid gas molar ratio, (CO+H₂)/(CO₂+H₂S), in thefeed to the fermentor. For example, the acid gas molar ratio in the feedto the fermentor may be controlled to a value selected from about 2 toabout 10 or more, or from about 10 to about 20.

In various embodiments, the tail gas contains between about 2% and about10% of the syngas contained in the raw syngas stream. In someembodiments, tail gas recycle improves mass transfer within thefermentor. In these or other embodiments, compressed tail gas recycleincreases the pressure within the fermentor, thereby allowing moresyngas to enter the liquid phase for bioconversion.

A reformer may be disposed between the gasifier and the acid-gas removalstep. The reformer may be utilized to convert or crack methane, tars, orother components, and produce additional syngas for bioconversion.

In some embodiments of the invention, the total conversion of CO and H₂is at least 90%, more preferably at least 95%, and most preferably atleast 98%. Other embodiments do not necessarily attempt to maximizesyngas conversion, but rather optimize syngas conversion to productsrelative to plant energy needs.

In preferred embodiments, the total conversion of CO and H₂ is at leastfive percentage points higher than the total conversion of CO and H₂that is attained in a comparable method with R₁ and R₂ both equal to 0.In more-preferred embodiments, the total conversion of CO and H₂ is atleast ten or fifteen percentage points higher than the total conversionof CO and H₂ that is attained in a comparable method with R₁ and R₂ bothequal to 0.

These methods may further include recovering the syngas-fermentationproduct from the fermentor. In some embodiments, the syngas-fermentationproduct is ethanol. The invention is, however, by no means limited toethanol. Another exemplary syngas-fermentation product is butanol, suchas 1-butanol.

Other variations of this invention provide an apparatus for converting acarbonaceous feedstock to a syngas-fermentation product, the apparatuscomprising:

(a) a gasifier for gasifying a carbonaceous feedstock with an oxidant,for producing a raw syngas stream comprising CO, H₂, and CO₂;

(b) an optional syngas-cleanup unit in communication with the gasifier,for producing an intermediate syngas stream from at least a portion ofthe raw syngas stream;

(c) an acid-gas removal unit in communication with the syngas-cleanupunit, if present; or in communication with the gasifier, if nosyngas-cleanup unit is present; for removing at least some of the CO₂and producing a conditioned syngas stream;

(d) a fermentor in communication with the acid-gas removal unit, forbiologically converting one or more of CO, H₂, or CO₂ to asyngas-fermentation product;

(e) a tail gas conduit in communication with the fermentor; and

(f) a recycle conduit in communication with the tail gas conduit forrecycling tail gas to the fermentor.

Still other variations of this invention provide an apparatus forconverting a carbonaceous feedstock to a syngas-fermentation product,the apparatus comprising:

(a) a gasifier for gasifying a carbonaceous feedstock with an oxidant,for producing a raw syngas stream comprising CO, H₂, and CO₂;

(b) an optional syngas-cleanup unit in communication with the gasifier,for producing an intermediate syngas stream from at least a portion ofthe raw syngas stream;

(c) an acid-gas removal unit in communication with the syngas-cleanupunit, if present; or in communication with the gasifier, if nosyngas-cleanup unit is present; for removing at least some of the CO₂and producing a conditioned syngas stream;

(d) a fermentor in communication with the acid-gas removal unit, forbiologically converting one or more of CO, H₂, or CO₂ to asyngas-fermentation product;

(e) a tail gas conduit in communication with the fermentor; and

(f) a recycle conduit in communication with the tail gas conduit forrecycling tail gas to the acid gas removal unit.

Yet other variations of this invention provide an apparatus forconverting a carbonaceous feedstock to a syngas-fermentation product,the apparatus comprising:

(a) a gasifier for gasifying a carbonaceous feedstock with an oxidant,for producing a raw syngas stream comprising CO, H₂, and CO₂;

(b) an optional syngas-cleanup unit in communication with the gasifier,for producing an intermediate syngas stream from at least a portion ofthe raw syngas stream;

(c) an acid-gas removal unit in communication with the syngas-cleanupunit, if present; or in communication with the gasifier, if nosyngas-cleanup unit is present; for removing at least some of the CO₂and producing a conditioned syngas stream;

(d) a fermentor in communication with the acid-gas removal unit, forbiologically converting one or more of CO, H₂, or CO₂ to asyngas-fermentation product;

(e) a tail gas conduit in communication with the fermentor;

(f) a recycle conduit in communication with the tail gas conduit forrecycling tail gas, wherein the recycle conduit includes a first conduitfor recycling a first amount of the tail gas to the fermentor and asecond conduit for recycling a second amount of the tail gas to theacid-gas removal unit.

The gasifier may be a fluidized-bed gasifier, for example. Someapparatus further include a reformer disposed between the gasifier andthe acid-gas removal unit. Preferred apparatus include one or morecompressors in communication with the recycle conduit. Some apparatusfurther include a purification unit for recovering, in purified form,the syngas-fermentation product from the fermentor.

The syngas-fermentation product may be ethanol, butanol, acetic acid,butyric acid, or any other biological products associated withproduction or growth of one or more microorganisms capable of consumingCO, H₂, and/or CO₂.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block-flow diagram depicting some embodiments of the presentinvention.

FIG. 2 is a block-flow diagram depicting some embodiments.

FIG. 3 is a block-flow diagram depicting certain embodiments.

FIG. 4 is a block-flow diagram depicting some embodiments of thisinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention, including what ispresently believed to be the best mode of carrying out the invention.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. For example, “a fermentor” includes aplurality of actual fermentors, in series or in parallel. Unless definedotherwise, all technical and scientific terms used herein have the samemeaning as is commonly understood by one of ordinary skill in the art towhich this invention belongs.

Unless otherwise indicated, all numbers expressing reaction conditions,stoichiometries, concentrations of components, and so forth used in thespecification and claims are to be understood as being modified in allinstances by the term “about.” Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the followingspecification and attached claims are approximations that may varydepending at least upon a specific analytical technique.

Some variations of the invention can be described by reference to theprocess configuration shown in FIG. 1, which relates to both apparatusand methods to carry out the invention. Any reference to a method “step”includes reference to an apparatus “unit” or equipment that is suitableto carry out the step, and vice-versa.

In the syngas-generation step, carbonaceous feedstock such as biomass isgasified with one or more oxidants to produce a raw syngas streamcomprising at least syngas (CO and H₂). Other gas species in the rawsyngas stream may include acid gases CO₂ and H₂S, relatively inertspecies such as CH₄ and N₂, and trace constituents such as tars, ash,and particulates.

The raw syngas stream from syngas generation may undergo one or moreclean-up steps to remove specific contaminants, such as particulates,thereby forming an intermediate syngas stream. The raw syngas streamand/or the intermediate syngas stream (which may include some amount ofrecycle) optionally undergo an acid-gas removal step to remove bulkamounts of CO₂ and/or H₂S, thereby forming a conditioned syngas stream.Typically, at least CO₂ (and H₂O) will be removed in an acid-gas removalunit, but H₂S removal may also be desired. Whether H₂S should also beremoved, and to what extent, typically depends on how much sulfur ispresent (if any) in the carbonaceous feedstock, the impact ofsulfur-containing compounds on downstream operations, and the impact H₂Sremoval may have on CO₂ removal.

The intermediate stream, upstream of the addition of a recycle stream(if any), will typically have between about 5-30 vol % CO₂. Theconditioned syngas stream, upstream of the addition of a recycle stream(if any), will typically have between about 1-25 vol % CO₂, or 2-20 vol% CO₂ in some embodiments. The tail gas stream, in various recyclescenarios, will typically have between about 10-90 vol % CO₂, such asabout 20-80 vol % CO₂, or about 25-75 vol % CO₂. Other ranges of CO₂content in various streams are possible, depending on many factors.

The conditioned syngas stream is suitable for direct biologicalconversion processes, wherein microorganisms (such as the microorganismsdescribed herein) directly convert one or more of H₂, CO, and CO₂ toethanol, acetic acid, butyric acid, butanol, or another fermentationproduct. When tail gas comprising syngas is recycled, the syngas isgiven another pass for biological conversion to ethanol or anotherproduct.

In some variations, as depicted in FIG. 1, at least a portion of thetail gas may be recycled to the fermentor feed, or to a CO₂-removal stepupstream of the fermentor feed, or to both of these locations. When aCO₂-removal unit is already in place, recycling to it is particularlyadvantageous because additional unit operations become unnecessary.

Some variations of the invention are premised on the realization thatrecycle streams can be tuned so that syngas generation andbalance-of-plant capital per unit product produced may actuallydecrease. With continued reference to FIG. 1, R₁ is the ratio of tailgas recycle to the fermentor feed divided by the total tail gas flow,each on a volume basis. R₂ is the ratio of tail gas recycle to theacid-gas removal unit divided by the total tail gas flow, each on avolume basis.

Recycle ratios R₁ and R₂ are non-negative numbers from 0 to 1. The sumof R₁+R₂ cannot exceed unity. R₁+R₂=1 represents total recycle of thetail gas, while R₁+R₂=0 represents no recycle of the tail gas to eitherlocations indicated in FIG. 1. By mass balance, the fraction of tail gasthat is not recycled plus R₁ plus R₂ must equal 1.

R₁ may be selected from various values from 0 to about 1, preferablyfrom 0 to about 0.5, and more preferably from 0 to about 0.2. R₂ may beselected from various values from 0 to about 1, preferably from about0.2 to about 0.8, and more preferably from about 0.2 to about 0.5. Thesum R₁+R₂ may be selected from various values greater than 0 (e.g.,0.001 or more) to about 1, preferably from about 0.2 to about 0.8, andmore preferably from about 0.25 to about 0.5.

R₁ should not be equal or close to one at steady state, because totalrecycle of tail gas back to the fermentor will cause a buildup of CO₂,other inerts, and syngas. However, in certain dynamic situations or dueto equipment problems (e.g., problems with the tail gas combustionunit), it is possible to recycle all of the tail gas back to thefermentor feed (R₁=1) for some amount of time.

R₂ should also generally not be equal or close to one at steady state,unless the acid-gas removal unit is functionally designed to also removeinerts (e.g., CH₄ or N₂) and anything else that must be purged somewherefrom the system. Again, in certain dynamic situations, it is possible toallow total recycle of tail gas to the acid-gas removal unit from someamount of time. These dynamic situations could include downstreamequipment problems, availability issues with feed streams in theprocess, fermentation issues (e.g., a stationary phase wherein syngasconversion drops significantly), and so on.

The recycle ratios R₁ and R₂ may be subjected to various means ofdynamic or steady-state process control. As is known, many feedforwardand feedback control strategies are possible. R₁ and R₂ mayindependently be set to control points for a desired steady state, orfor a desired or known unsteady state. A person of skill in the art ofprocess control will also understand that the ratio of R₁ to R₂,derivatives of R₁ and R₂ with time, the ratio of the time derivatives ofR₁ and R₂, the derivatives of process variables (such as CO or H₂conversion, or ethanol productivity) with R₁ and R₂, and so on, may beutilized in various control strategies.

The following are exemplary control examples only and should not beconstrued as limiting in any way, or as being related to any particularfundamentals being applied. These examples demonstrate that R₁ and/or R₂can be set to vary over time or as a function of other conditions in theprocess.

In some embodiments, R₁ and/or R₂ are adjusted continuously, or at leastdynamically (e.g., periodically or intermittently), in response to oneor more upstream parameters such as feedstock type, oxidant profile,syngas-generation design or performance, syngas-cleanup design orperformance, or acid-gas removal design or performance.

In some embodiments, R₁ and/or R₂ are adjusted continuously, or at leastdynamically (e.g., periodically or intermittently), in response to oneor more fermentor parameters such as temperature, pressure, residencetime, pH, redox potential, nutrient concentration, microorganismviability or vitality, and so on.

In some embodiments, R₁ and/or R₂ are adjusted to one or more fermentordesign or performance variables such as CO conversion, H₂ conversion,CO₂ conversion, ethanol selectivity, ethanol productivity, ethanoltiter, or acetic acid selectivity. Such adjustment may be in combinationwith a response to fermentor parameter, such as those listed above.

Certain embodiments adjust R₁ and/or R₂ to change or optimize the CO₂content in the fermentor feed. The CO₂ level in the fermentor feed canbe varied, by adjusting R₁ and/or R₂, to about 5-50 vol % CO₂, such asabout 10-40 vol % CO₂, or about 20-30 vol % CO₂. Certain embodimentsincrease R₂, relative to R₁, so that more CO₂ can be removed in theacid-gas removal step and decrease the CO₂ level in the fermentor feed.

Some embodiments adjust R₁ and/or R₂ to change or optimize the syngas toacid gas molar ratio, (CO+H₂)/(CO₂+H₂S), at one or more points in theprocess. Certain, preferred embodiments adjust R₁ and/or R₂ to change oroptimize the syngas to acid gas molar ratio, (CO+H₂)/(CO₂+H₂S), in thefeed stream entering the fermentor. The syngas to acid gas molar ratioentering the fermentor can be varied, by adjusting R₁ and/or R₂, betweenabout 2 to about 10 or more, such as about 11, 12, 13, 14, 15, 16, 17,18, 19, or 20.

The syngas feed to the fermentor is typically at a higher pressure thanthe tail gas pressure. The reason is that upstream operations(gasification and acid-gas removal) generally favor higher pressurescompared to fermentation. For example, the feed pressure to thefermentor may be about 2-40 barg, while the pressure of the tail gas maybe about 0.1-2 barg (usually not greater than 1 barg). In order torecycle a gas stream to an upstream point that is at higher pressure,the pressure of the gas stream being recycled needs to be raised. Ratherthan removing CO₂ from the tail gas, compressing the remainder, and thenrecycling it back to the fermentor, this invention contemplatesrecycling some portion of the tail gas and compressing it, withoutotherwise separating its components. That is, a “portion” of the tailgas stream in this context refers to a flow split only, by someflow-splitting means (e.g., valves)—not a component split by someseparation means.

In FIG. 1, the recycled tail gas is compressed upstream of the R₁/R₂split. In other embodiments, the recycled tail gas may be split into twoor more recycle streams and then each of these streams compressed. Whilethis adds some cost, it allows for adjusting the pressure increase inthe recycle streams individually, if desired.

The amount of compression may be varied, but the pressure of a recyclestream should be at least raised to a pressure sufficient to allow itsintroduction into the stream(s) of interest. It is possible to compressthe recycled tail gas, particularly when recycled back to the acid-gasremoval unit, such that the pressure of the combined stream is actuallyincreased. This would add operating costs but may improve the CO₂removal.

In some embodiments, the conversion of syngas is about 90-98% (molarconversion of CO and H₂). The syngas conversion may be influenced by anumber of factors, including the levels of inerts in the conditionedsyngas stream, and the fermentor conditions, such as temperature, pH,mixing and mass transfer, the presence of competing microorganisms, andso on. In some embodiments, the syngas conversion is 90-98% uponrecycling of tail gas as described herein, and less than 90% (such asonly 75-85%) without tail gas recycling, all other factors being heldconstant. Preferably, syngas conversion is about 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15 or more percentage points higher byimplementing one or more of the recycle methods taught herein.

Higher overall syngas conversion will mean that the tail gas containsless of the syngas initially generated. In some embodiments with tailgas recycle, the tail gas contains about 2% to about 10% of the syngascontained in the raw syngas stream; whereas, without tail gas recycle(R₁, R₂=0), the tail gas contains about 10% to about 25% of the syngascontained in the raw syngas stream. The syngas concentration and energycontent of the tail gas stream is not necessarily less when tail gasrecycle is employed, because CO₂ can be removed from the acid-gasremoval step. The non-recycled tail gas flow rate may be reduced, insome embodiments.

Higher syngas conversions will translate into higher yields of productsof interest, such as ethanol, because product selectivity is notexpected to decrease using these recycle strategies. Product selectivitymay actually improve when less CO₂ is fed to the fermentor, furtherincreasing product yield.

FIGS. 2-4 are provided to indicate other variations of the invention. InFIG. 2, the carbonaceous feedstock is biomass, the oxidant isoxygen-enriched air, and the product of interest is ethanol. In FIG. 3,there is recycle of some of the tail gas to the fermentor, but not anyrecycle to the acid-gas removal unit (R₂=0). In FIG. 4, there is recycleof some of the tail gas to the acid-gas removal unit, but not anyrecycle to the fermentor (R₁=0). All other aspects of theseconfigurations may be selected or characterized as described withreference to FIG. 1 herein.

The syngas-generation unit or step may be selected from any known means,such as a gasifier. The gasifier could be, but is not limited to, afluidized bed. Any known means for devolatilization or gasification canbe employed. In variations, the gasifier type may be entrained-flowslagging, entrained flow non-slagging, transport, bubbling fluidizedbed, circulating fluidized bed, or fixed bed. Some embodiments employknown gasification catalysts. “Gasification” and “devolatilization”generally refer herein to the reactive generation of a mixture of atleast CO, CO₂, and H₂, using oxygen, air, and/or steam as theoxidant(s).

If gasification is incomplete, a solid stream can be generated,containing some of the carbon initially in the feed material. The solidstream produced from the gasification step can include ash, metals,unreacted char, and unreactive refractory tars and polymeric species.Generally speaking, feedstocks such as biomass contain non-volatilespecies, including silica and various metals, which are not readilyreleased during pyrolysis, devolatilization, or gasification. It is ofcourse possible to utilize ash-free feedstocks, in which case thereshould not be substantial quantities of ash in the solid stream from thegasification step.

When a fluidized-bed gasifier is employed as the devolatilization unit,the feedstock can be introduced into a bed of hot sand fluidized by agas. Reference herein to “sand” shall also include similar,substantially inert materials, such as glass particles, recovered ashparticles, and the like. High heat-transfer rates from fluidized sandcan result in rapid heating of the feedstock. There can be some ablationby attrition with the sand particles. Heat is usually provided byheat-exchanger tubes through which hot combustion gas flows.

Circulating fluidized-bed reactors can be employed as thedevolatilization unit, wherein gas, sand, and feedstock move together.Exemplary transport gases include recirculated product gas, combustiongas, or recycle gas. High heat-transfer rates from the sand ensure rapidheating of the feedstock, and ablation is expected to be stronger thanwith regular fluidized beds. A separator may be employed to separate theproduct gases from the sand and char particles. The sand particles canbe reheated in a fluidized burner vessel and recycled to the reactor.

In some embodiments in which a countercurrent fixed-bed reactor is usedas the gasifier, the reactor consists of a fixed bed of a feedstockthrough which a gasification agent (such as steam, oxygen, and/orrecycle gas) flows in countercurrent configuration. The ash is eitherremoved dry or as a slag.

In some embodiments in which a cocurrent fixed-bed reactor is used asthe gasifier, the reactor is similar to the countercurrent type, but thegasification agent gas flows in cocurrent configuration with thefeedstock. Heat is added to the upper part of the bed, either bycombusting small amounts of the feedstock or from external heat sources.The produced gas leaves the reactor at a high temperature, and much ofthis heat is transferred to the gasification agent added in the top ofthe bed, resulting in good energy efficiency. Since tars pass through ahot bed of char in this configuration, tar levels are expected to belower than when using the countercurrent type.

In some embodiments in which a fluidized-bed reactor is used as thegasifier, the feedstock is fluidized in recycle gas, oxygen, air, and/orsteam. The ash is removed dry or as heavy agglomerates that defluidize.Recycle or subsequent combustion of solids can be used to increaseconversion. Fluidized-bed reactors are useful for feedstocks that formhighly corrosive ash that would damage the walls of slagging reactors.

The primary fluidizing agent for a fluidized-bed gasifier may be recyclegas, possibly including a portion of the fermentor tail gas. Due to thehigh heat-transfer characteristics of a fluidized bed, the recycle gaswill cool and give up a portion of its sensible heat to thecarbon-containing feedstock particles. Utilizing hot recycle gas tofluidize a bed of incoming biomass particles leads to improved overallenergy efficiency.

In some embodiments in which an entrained-flow reactor is used as thegasifier, char is gasified with oxygen, air, or recycle gas in cocurrentflow. The gasification reactions take place in a dense cloud of veryfine particles. High temperatures may be employed, thereby providing forlow quantities of tar and methane in the product gas.

Entrained-flow reactors remove a major part of the ash as a slag, as theoperating temperature is typically well above the ash fusiontemperature. A smaller fraction of the ash is produced either as a veryfine dry fly ash or as a fly-ash slurry. Some feedstocks, in particularcertain types of biomass, can form slag that is corrosive. Certainentrained-bed reactors have an inner water- or steam-cooled wall coveredwith partially solidified slag.

In certain embodiments, the process configuration further includes areformer disposed between the gasifier and the optional syngas-cleanupstep or the acid-gas removal step. The reformer may be employed toconvert or crack tars and methane to produce additional syngas, in someembodiments, optionally with a reforming catalyst.

The optional reformer, which can be regarded as within thesyngas-generation unit of FIGS. 1-4, is any reactor capable of causingat least one chemical reaction that produces syngas. Conventional steamreformers, well-known in the art, may be used either with or without acatalyst. Other possibilities include autothermal reformers,partial-oxidation reactors, and multistage reactors that combine severalreaction mechanisms (e.g., partial oxidation followed by water-gasshift). The reactor configuration may be a fixed bed, a fluidized bed, aplurality of microchannels, or some other configuration.

Heat can be supplied to the reformer reactor in many ways including, forexample, by oxidation reactions resulting from oxygen added to theprocess. In some embodiments, a direct-fired partial-oxidation reactoris employed, wherein both oxygen and fuel are directly injected into thereactor to provide heat and assist in reforming and cracking reactions.

The reformer may include homogeneous (non-catalyzed) partial oxidation,catalytic partial oxidation, or both. Steam-reforming reactions may alsobe catalyzed. Reforming and/or partial-oxidation catalysts include, butare not limited to, nickel, nickel oxide, nickel alloys, rhodium,ruthenium, iridium, palladium, and platinum. Such catalysts may becoated or deposited onto one or more support materials, such as, forexample, gamma-alumina (optionally doped with a stabilizing element suchas magnesium, lanthanum, or barium).

When a reformer is employed, the gasifier chamber can be designed, byproper configuration of the freeboard or use of internal cyclones, tokeep the carryover of solids to the downstream reformer at a levelsuitable for recovery of heat downstream of the reformer. Unreacted charcan be drawn from the bottom of the devolatilization chamber, cooled,and then fed to a utility boiler to recover the remaining heating valueof this stream.

The syngas-cleanup unit is not particularly limited in its design.Exemplary syngas-cleanup units include cyclones, centrifuges, filters,membranes, solvent-based systems, and other means of removingparticulates and/or other specific contaminants.

The acid-gas removal unit is also not particularly limited, and may beany means known in the art for removing at least CO₂ from syngas.Examples include removal of CO₂ with one or more solvents for CO₂, orremoval of CO₂ by a pressure-swing adsorption unit. Suitable solventsfor reactive solvent-based acid gas removal include monoethanolamine,diethanolamine, methyldiethanolamine, diisopropylamine, andaminoethoxyethanol. Suitable solvents for physical solvent-based acidgas removal include dimethyl ethers of polyethylene glycol (such as inthe Selexol® process) and refrigerated methanol (such as in theRectisol® process).

Bioconversion of CO or H₂/CO₂ to acetic acid, ethanol, or other productsis well known. For example, syngas biochemical pathways and energeticsof such bioconversions are summarized by Das and Ljungdahl, “ElectronTransport System in Acetogens” and by Drake and Kusel, “DiversePhysiologic Potential of Acetogens,” appearing respectively as Chapters14 and 13 of Biochemistry and Physiology of Anaerobic Bacteria, L. G.Ljungdahl eds,. Springer (2003).

Any suitable microorganisms may be utilized that have the ability toconvert CO, H₂, or CO₂, individually or in combination with each otheror with other components that are typically present in syngas. Theability of microorganisms to grow on CO as their sole carbon source wasfirst discovered over one hundred years ago. A large number of anaerobicorganisms including carboxydotrophic, photosynthetic, methanogenic, andacetogenic organisms have been shown to metabolize CO to various endproducts.

Anaerobic bacteria, such as those from the genus Clostridium, have beendemonstrated to produce ethanol from CO, H₂, or CO₂ via the acetyl CoAbiochemical pathway. For example, various strains of Clostridiumljungdahlii that produce ethanol from gases are described in U.S. Pat.Nos. 5,173,429, 5,593,886, and 6,368,819. The bacterium Clostridiumautoethanogenum sp is also known to produce ethanol from gases (Aribiniet al., Archives of Microbiology 161, pp. 345-351 (1994)).

Generally speaking, microorganisms suitable for syngas fermentation inthe context of the present invention may be selected from many generaincluding Clostridium, Moorella, Carboxydothermus, Acetogenium,Acetobacterium, Butyribacterium, Peptostreptococcus, and Geobacter.Microorganism species suitable for syngas fermentation in this inventionmay be selected from Clostridium ljungdahlii, Clostridiumautoethanogenum, Clostridium ragsdalei, Clostridium carboxidivorans,Butyribacterium methylotrophicum, Eubacterium limosum, and geneticallyengineered, mutated, or evolved variations thereof. Microorganisms thatare engineered, created, or provided in the future will be applicable tothe present invention, provided such new microorganisms can convert oneor more of CO, H₂, or CO₂ to a product of interest.

A selected microorganism may be grown, to at least some extent, in thefermentor itself (simultaneous growth and production) or may be grown ina separate growth vessel or train. When separate cell growth isutilized, microorganism cells can be grown from any carbon substrate,which could be syngas but also could be various sugars such as glucose,galactose, arabinose, xylose, mannose, and other C₅ or C₆ sugars.

The fermentor, or plurality of fermentors (in series or parallel), isnot particularly limited but will generally be selected from amechanically agitated reactor, a bubble column, a packed column, a platecolumn, a spray column, a gas-lift reactor, and a membrane reactor. Insome embodiments, gas or liquid internal recycle is utilized to add masstransfer within the fermentor. Surfactants, water co-solvents, andmicrobubbles may all be utilized in various embodiments to enhancemixing and mass transfer.

In certain embodiments, tail gas recycle improves mass transfer withinthe fermentor. In certain embodiments, compressed tail gas recycleincreases the pressure within the fermentor, thereby allowing moresyngas to enter the liquid phase for bioconversion.

Some embodiments employ cell recycle back to the fermentor. Someembodiments employ recycle of cells, or fermentation sludge, back to thegasifier. Sludge recycling allows for conversion of used microorganismsback to syngas.

The mechanical art necessary for implementing the tail gas recyclestreams is well established. With reference to FIG. 1, which isnon-limiting, what is needed is a flow splitter in the tail gas stream,at least one compressor, a flow splitter to adjust R₁ and R₂, andappropriate pipes and valves.

The compressor is not limited but should be a mechanical device thatincreases the pressure of the tail gas by reducing its volume. Suitablecompressors include centrifugal compressors, diagonal compressors,axial-flow compressors, reciprocating compressors, rotary screwcompressors, rotary vane compressors, scroll compressors, and diaphragmcompressors.

The methods and apparatus of the invention can accommodate a wide rangeof feedstocks of various types, sizes, and moisture contents. “Biomass,”for the purposes of the present invention, is any material not derivedfrom fossil resources and comprising at least carbon, hydrogen, andoxygen. Biomass includes, for example, plant and plant-derived material,vegetation, agricultural waste, forestry waste, wood waste, paper waste,animal-derived waste, poultry-derived waste, and municipal solid waste.Other exemplary feedstocks include cellulose, hydrocarbons,carbohydrates or derivatives thereof, and charcoal.

In various embodiments of the invention utilizing biomass, the biomassfeedstock can include one or more materials selected from: timberharvesting residues, softwood chips, hardwood chips, tree branches, treestumps, leaves, bark, sawdust, off-spec paper pulp, corn, corn stover,wheat straw, rice straw, sugarcane bagasse, switchgrass, miscanthus,animal manure, municipal garbage, municipal sewage, commercial waste,grape pumice, almond shells, pecan shells, coconut shells, coffeegrounds, grass pellets, hay pellets, wood pellets, cardboard, paper,plastic, and cloth.

The present invention can also be used for carbon-containing feedstocksother than biomass, such as a fossil fuel (e.g., coal or petroleumcoke), or any mixtures of biomass and fossil fuels. For the avoidance ofdoubt, any method, apparatus, or system described herein can be usedwith any carbonaceous feedstock.

Selection of a particular feedstock or feedstocks is not regarded astechnically critical, but is carried out in a manner that tends to favoran economical process. Typically, regardless of the feedstocks chosen,there is screening to remove undesirable materials. The feedstock canoptionally be dried prior to processing. Optionally, particle-sizereduction can be employed prior to conversion of the feedstock tosyngas. Particle size is not, however, regarded as critical to theinvention.

When multiple feedstocks are used (e.g., biomass-coal mixtures), theymay be used in any ratio and they may be introduced in the same ordifferent locations. It will be understood that the specific selectionof feedstock ratios can be influenced by many factors, includingeconomics (feedstock prices and availability), process optimization(depending on feedstock composition profiles), utility optimization,equipment optimization, and so on.

A variety of operating temperatures, pressures, flow rates, andresidence times can be employed for each unit operation of FIGS. 1-4 orother variations of the invention. As is known to a skilled artisan, theoptimum conditions for each unit will be influenced by the conditions ofother units.

Some embodiments of the invention relate to integration with the plantenergy balance. The recycle loop(s) as described may be implemented tocontrol the conversion of syngas to ethanol, adjusting for asteady-state or dynamic energy demand for syngas as an energy source.This invention allows real-time adjustment of how syngas is utilized inthe overall process, thereby enhancing plant efficiency and economics.

In general, solid, liquid, and gas streams produced or existing withinthe process can be independently passed to subsequent steps orremoved/purged from the process at any point. Also, any of the streamsor materials present may be subjected to additional processing,including heat addition or removal, mass addition or removal, mixing,various measurements and sampling, and so forth.

In this detailed description, reference has been made to multipleembodiments of the invention and non-limiting examples relating to howthe invention can be understood and practiced. Other embodiments that donot provide all of the features and advantages set forth herein may beutilized, without departing from the spirit and scope of the presentinvention. This invention incorporates routine experimentation andoptimization of the methods and systems described herein. Suchmodifications and variations are considered to be within the scope ofthe invention defined by the claims.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain of the steps may be performedconcurrently in a parallel process when possible, as well as performedsequentially.

Therefore, to the extent there are variations of the invention, whichare within the spirit of the disclosure or equivalent to the inventionsfound in the appended claims, it is the intent that this patent willcover those variations as well. The present invention shall only belimited by what is claimed.

What is claimed is:
 1. A method of converting a carbonaceous feedstockto a syngas-fermentation product, said method comprising: (a)introducing a carbonaceous feedstock and an oxidant to a gasifier, undersuitable gasification conditions to produce a raw syngas streamcomprising CO, H₂, and CO_(2;) (b) optionally feeding at least a portionof said raw syngas stream to a syngas-cleanup unit, to produce anintermediate syngas stream; (c) feeding at least a portion of said rawsyngas stream and/or at least a portion of said intermediate syngasstream, if present, to an acid-gas removal unit, to remove at least someof said CO₂ and produce a conditioned syngas stream; (d) feeding atleast a portion of said conditioned syngas stream to a fermentor, undersuitable fermentation conditions and in the presence of suitablemicroorganisms and nutrients to biologically convert one or more of CO,H₂, or CO₂ to a fermentation product and generate a product streamcomprising at least the fermentation product and a tail gas streamcomprising at least CO₂ and unconverted CO or H₂; (e) recycling at leasta first portion of the tail gas to the fermentor, at least a secondportion of the tail gas to the acid-gas removal unit, or both.
 2. Themethod of claim 1 wherein no tail gas is recycled to the fermentor for aperiod of time.
 3. The method of claim 1 wherein no tail gas is recycledto the acid-gas removal unit for a period of time.
 4. The method ofclaim 1, wherein said carbonaceous feedstock comprises biomass, fossilfuels, or any combination thereof.
 5. The method of claim 1, whereinsaid oxidant comprises one or more of air, oxygen, and steam.
 6. Themethod of claim 1, wherein said gasifier is a fluidized-bed gasifier,bubbling fluidized bed gasifier, a circulating fluidized bed gasifier, afixed bed gasifier, an entrained-flow slagging gasifier, entrained flownon-slagging gasifier, or a transport gasifier.
 7. The method of claim1, comprising feeding at least a portion of said raw syngas stream to asyngas-cleanup unit, to produce an intermediate syngas stream.
 8. Themethod of claim 1, wherein said acid-gas removal unit additionallyremoves at least some H₂S, if present.
 9. The method of claim 1, furthercomprising compressing the first portion of the tail gas recycled to thefermentor, the second portion of the tail gas recycled to the acid-gasremoval unit, or both.
 10. The method of claim 1, further comprisingcontrolling the amounts the first portion of the tail gas recycled tothe fermentor and the second portion of the tail gas recycled to theacid-gas removal unit based on one or more upstream parameters selectedfrom feedstock type, oxidant profile, syngas-generation design orperformance, syngas-cleanup design or performance, acid-gas removaldesign or performance.
 11. The method of claim 1, further comprisingcontrolling the amounts the first portion of the tail gas recycled tothe fermentor and the second portion of the tail gas recycled to theacid-gas removal unit based on one or more fermentor parameters selectedfrom temperature, pressure, residence time, pH, redox potential,nutrient concentration, cell viability, and cell vitality.
 12. Themethod of claim 1, further comprising controlling the amounts the firstportion of the tail gas recycled to the fermentor and the second portionof the tail gas recycled to the acid-gas removal unit based on one ormore fermentor variables selected from the group consisting of COconversion, H₂ conversion, CO₂ conversion, ethanol selectivity, ethanolproductivity, ethanol titer, and acetic acid selectivity.
 13. The methodof claim 1, further comprising controlling the amounts the first portionof the tail gas recycled to the fermentor and the second portion of thetail gas recycled to the acid-gas removal unit to control the CO₂content in the feed to said fermentor.
 14. The method of claim 1,further comprising controlling the amounts the first portion of the tailgas recycled to the fermentor and the second portion of the tail gasrecycled to the acid-gas removal unit to control the acid gas molarratio, (CO+H₂)/(CO₂+H₂S), in the feed to said fermentor.
 15. The methodof claim 1, further comprising compressing the first portion of the tailgas recycled to the fermentor, the second portion of the tail gasrecycled to the acid-gas removal unit, or both to increase the pressurewithin said fermentor and increase the amount of syngas entering theliquid phase for bioconversion.
 16. The method of claim 1, furthercomprising a reformer disposed between said gasifier and said acid-gasremoval step.
 17. The method of claim 1, further comprising recyclingcells from said fermentor back to said fermentor, said gasifier, orboth.
 18. The method of claim 1, wherein the total conversion of CO andH₂ is at least five percentage points higher than the total conversionof CO and H₂ that is attained in a comparable method without recyclingat least a first portion of the tail gas to the fermentor and withoutrecycling at least a second portion of the tail gas to the acid-gasremoval unit.